Find Wet Ground with the Topographic Wetness Index
The corner of the field that is still soft in June, the low spot where the snowmelt sits, the bench that dries first. Map where the shape of the land keeps a Wisconsin property wet, from USGS 3DEP lidar.
Map a property now
Select the parcel and run the Wetness Map. Tan is dry, deep blue is where water gathers.
Open the Wetness Map →Why the shape of the land matters
Water runs downhill and slows where the ground flattens. Ground that has a lot of land draining into it and little slope to carry the water away stays wet longer: the floor of a draw, the foot of a long slope, a flat below a hillside. Ground with nothing above it and plenty of fall, a ridge top or a spur, dries out first.
The Wetness Map puts that into a number for every cell, the topographic wetness index, and colors the property by it. In the kettles and swales of the Kettle Moraine, on the flat ground of central Wisconsin or in the hollows of a Driftless hillside, it shows where water will gather before you have stood in it.
What people use it for
- Buying land. See how much of a parcel sits in the wettest ground before a spring visit tells you the hard way.
- Siting a house or a shop. Keep the building, the driveway and the basement out of the blue ground.
- Screening a drain field. Rule out the wettest ground early, before soil tests, which decide the question. Septic rules are set by Wisconsin DSPS and administered by your county.
- Farming. Target drainage tile, choose where to graze and where to keep machinery off in a wet spring.
- Planting and restoration. Wet-tolerant species in the blue, drought-tolerant on the tan.
How to map wet ground
- Select the area. Click the parcel, draw a box or polygon, or upload a boundary. Without a selection, Analyze this view uses the area on screen.
- Open the Wetness Map and choose Run analysis. Water is routed downhill across the terrain, with a 0.62 mi buffer loaded around the area so that upslope ground beyond the boundary counts.
- Read the colors. Tan is the driest ground, deep blue the wettest. The color bar and the map legend show the same two end values.
- Read the panel. Minimum, mean and maximum are the true extremes of the index across the area.
Reading the map honestly
It is a ranking, not a reading
The colors are stretched between the 2nd and 98th percentile of the area, so the deepest blue always means the wettest ground in this area. On a dry sandy ridge parcel there is still a deepest blue. Use the map to rank ground within one property, not to compare two places run separately.
Terrain only
The index knows the shape of the ground, nothing else. Soil, the water table, springs, drainage tile and ditches all change how wet ground really is. Sandy soil and heavy clay behave very differently on the same shape of land.
Not a wetland determination
Whether part of a property is a regulated wetland is a separate question, answered from soils, hydrology and plants. Start with the Wisconsin DNR wetlands information and your county before you plan any work in the blue ground.
Follow the water further
The wetness map shows where water gathers. These show where it goes and where it comes from.
About the data
Ground elevations come from the USGS 3D Elevation Program (3DEP) seamless bare-earth model, with elevations above NAVD 88. Source cells are 3.3 ft per pixel in the finest work units, with about 9.8 ft per pixel and 33 ft per pixel sources elsewhere. USGS elevation data is public domain. Lidar coverage and source dates vary across Wisconsin. Check your site against the USGS 3DEP work-unit index; an unavailable lookup means coverage is unknown. The index is computed on the bare-earth model, so a culvert under a road is not in the terrain and water is routed over the embankment instead. More from the USGS 3D Elevation Program and Wisconsin elevation resources.
Frequently asked questions
What is the topographic wetness index?
A standard hydrology measure of how much the shape of the land tends to gather water at a point. For each cell it takes the area of ground draining into it and divides by the local slope. A lot of upslope ground draining onto a flat spot gives a high value; a steep ridge with nothing above it gives a low one.
Is it a wetland map?
No. It ranks ground by terrain shape alone. Whether land is a wetland is decided from soils, water and vegetation, and the Wisconsin DNR is the place to start. Use the wetness map to see where to look, not to decide what you find.
Why are the values different on two properties?
The colors are stretched across the range of each run, so the deepest blue is the wettest ground in that area, not a fixed threshold. Values are comparable within one run, not between different areas. To compare two parcels, draw one area that covers both.
Does it know about my drainage tile and ditches?
No. Tile, ditches, culverts and soil type are not in the terrain model. A tiled field can rank wet and drain well; a field on heavy clay can rank dry and still hold water. Read the map with what you know of the ground.
Does water from the neighbor’s land count?
Yes. A 0.62 mi buffer of terrain is loaded around your area, so ground upslope of the boundary is counted when water is routed downhill.
Can I download the wetness map?
Yes, as a GeoTIFF for GIS, from the results panel. Downloading needs a pass; the map itself is free to run.
Related guides
- Drainage area to a culvert, pond or stream crossing — where the water arriving at a point comes from.
- Terrain checks before buying rural land — drainage as part of the pre-offer pass.
- Find the best house site on your land — the wetness test combined with slope and sun.